1. Field of the Invention
The present invention relates to a developing apparatus for use in an image forming apparatus such as an electrophotographic device or an electrostatic recording device.
2. Related Background Art
In an image forming apparatus in which an electrophotographic method is used, an electrostatic latent image is formed on an image bearing member and is developed to be visualized as a toner image, which is transferred onto a transfer material so as to obtain an image.
As a developing method in the above, there are generally a monocomponent developing method with monocomponent developer composed of magnetic toner only and a two-component developing method with two-component developer composed of magnetic particles (magnetic carrier) and nonmagnetic toner; the monocomponent developing method has merits such as a simple constitution of a developing apparatus and easy maintenance in comparison with the two-component method, and therefore various types of developing apparatuses for the monocomponent developing method have been suggested and put into practical use.
There is suggested a developing method with nonmagnetic monocomponent developer composed of nonmagnetic toner only as shown in Japanese Patent Application Laid-open No. 58-116559, and low-cost and small-sized developing apparatuses which are capable of generating high-quality color images have been put into practical use, so as to cope with an increase of color images in recent years.
An example of a non-magnetic monocomponent developing apparatus is shown in FIG. 6. As shown in FIG. 6, the developing apparatus has a developer container 3 containing non-magnetic toner of the non-magnetic monocomponent developer type, a developing sleeve 1 rotated as a developer bearing member disposed therein, and a toner supplying roller 14 as a developer supplying member and a regulating blade 13 as a developer regulating member abutting against the developing sleeve 1.
The toner supplying roller 14 is formed by a metal core 14a such as SUS coated by an elastic member 14b made of urethane foam on its outer peripheral surface and it rotates while sliding frictionally with the developing sleeve 1 to supply non-magnetic toner contained in the developer container 3 to a surface of the developing sleeve 1 and to scrape off nonmagnetic toner returned to the inside of the developer container 3 with the rotation of the developing sleeve 1 without contributing to the development in the developing section opposite to a photosensitive drum from the surface of the developing sleeve 1.
For the regulating blade 13, an elastic member 13b made of urethane rubber or the like is bonded in the opposite side of the developing sleeve 1 of a supporting member 13a made of phosphor bronze, and the elastic member 13b is put into contact with a surface of the developing sleeve 1 to regulate toner born on the developing sleeve 1 so as to form a thin toner layer and to triboelectricity charge the toner.
According to the constitution set forth in the above, the developing apparatus is capable of favorably forming a thin layer of charged non-magnetic toner on the developing sleeve 1 to supply the toner for development of an electrostatic latent image formed on a surface of the photosensitive drum, by which the latent image can be developed favorably.
In the above nonmagnetic monocomponent developing apparatus, however, the charge supply to the toner is mainly performed at a regulation of a thickness of the toner layer with the regulating blade 13, and the toner is charged by contact friction generated when the toner passes the regulating blade 13. In addition, the supplying roller 14 is made to abut against the developing sleeve 1 so as to rub the toner to supply it to the developing sleeve 1 while collecting toner on the developing sleeve 1.
Accordingly, in the non-magnetic monocomponent developing apparatus, an enormous mechanical load is applied on the toner in the developer container 3 and therefore damage to the toner is extremely large in comparison with other developing methods.
Furthermore, depending upon a disposed position or a rotary direction of the toner supplying roller 14, toner which has not contributed to development cannot be collected completely and sometimes toner remains on the developing sleeve 1. Although the residual toner on the developing sleeve 1 is carried to a developing zone passing the regulating blade 13 again, repetition of this carry-back (re-carry) causes outer additive or the like which controls a charged amount or flowability of toner to be buried into the inside of the toner due to a mechanical friction applied to the toner every time or an accumulated heat, by which the toner is deteriorated and desired electrification properties or flowability cannot be obtained.
This deteriorated toner causes a lot of problems in an image formation process. For example, if deteriorated toner is used for development, a development failure may occur since appropriate development properties cannot be obtained or a transfer hollow of an image may occur at a transfer operation. Furthermore, a new toner supply to the developing sleeve 1 is obstructed to lower a coating amount of toner on the developing sleeve 1, by which a poor density may occur. In addition, if deteriorated toner fuses in a nip portion between the regulating blade 13 and the developing sleeve 1 or on the surface of the developing sleeve 1, a toner coating failure such as a line on the developing sleeve 1 may occur and a charge supply to new toner supplied to the developing sleeve 1 may be obstructed. Accordingly, uncharged toner is carried to the developing zone, which may cause an image failure such as a fogging or unevenness.
Furthermore, in a nonmagnetic monocomponent developing apparatus, the enormous load is applied not only on toner, but also on the developing apparatus itself. For example, regarding the toner supplying roller 14, if it is formed into a sponge roller as described in the above, a rubbing friction applied to the developing sleeve 1 for a long time leads to wear or a damage of the toner supplying roller 14 itself or clogging of toner, and therefore toner cannot be favorably supplied nor collected.
As described above, the nonmagnetic monocomponent developing apparatus generates favorably developed images in spite of having a simple constitution, while an enormous load is applied to toner and the apparatus, lacking a long-term stability in comparison with a magnetic monocomponent developing apparatus and a two-component developing apparatus. Therefore, this method is mainly used for cartridge-type developing apparatuses which themselves are replaced with new ones when toner is used up and it is not often used for toner-replenishing type developing apparatuses.